1. Field of the Invention
The present invention relates to a method for preparing superconducting thin film, and more specifically to an improved MBE (Molecular Beam Epitaxy) process particularly for preparing superconducting thin film formed of a high temperature superconductor oxide, which has a clean surface, high crystallinity and excellent superconducting properties.
2. Description of Related Art
High temperature superconductor oxides (abbreviated to oxide superconductors hereinafter) have higher critical temperatures than those of metal superconductors, and therefore have been considered to have high possibility of practical use. For example, Y--Ba--Cu--O type oxide superconductor has a critical temperature higher than 80K and it is reported that Bi--Sr--Ca--Cu--O type oxide superconductor and Tl--Ba--Ca--Cu--O type oxide superconductor have critical temperatures higher than 100K. The oxide superconductor has a crystalline anisotropy in superconducting characteristics, and in particular, its critical current density is maximum in a direction perpendicular to a c-axis of crystal lattice. Therefore, when the oxide superconductor is used, attention should be paid to the crystalline orientation.
In case of applying the oxide superconductor to superconducting electronics including superconducting devices and superconducting integrated circuits, the oxide superconductor has to be used in the form of a thin film having a thickness from a few nanometers to some hundreds nanometers. When the oxide superconductor is in the form of a thin film, the above mentioned crystalline anisotropy of superconducting characteristics may become more remarkable. In addition, in order to realize high performance superconducting devices and superconducting integrated circuits, high crystalline oxide superconductor thin films should be necessary.
Oxide superconductor thin films are often deposited by various sputtering processes, or a laser ablation process. However, in the case of controlling a thickness of a depositing oxide superconductor thin film precisely or depositing an oxide superconductor thin film and thin films of other materials successively, an MBE process may be the most suitable.
The MBE process is one of a vacuum evaporation process in which a thin film is deposited on a substrate by molecular beams from plural vapor sources or molecular beam sources which are arranged opposite to the substrate in a vacuum chamber. A Knudsen's cell (abbreviated to K cell hereinafter), a crucible heated by an electric gun, a laser heating crucible etc. are used for the vapor sources or molecular sources. In the MBE process, composition of a depositing thin film is controlled by ratios of intensity of the molecular beams. The intensity of the molecular beams is controlled by temperatures of crucibles of the vapor sources or molecular sources.
In order to deposit an oxide superconductor thin film, simple metal vapor sources are used. For example, when Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-67 oxide superconductor thin film is deposited, a metal yttrium vapor source, a metal barium vapor source and a metal copper vapor source are used. Oxidizing gas such as O.sub.2 including O.sub.3, NO.sub.2 or N.sub.2 O is supplied near the substrate so that the metal molecular beams are oxidized so as to form the oxide superconductor thin film on the substrate.
By the MBE process, it is possible to control thickness of a depositing thin film, layer by layer, and to deposit thin films of different kinds successively, under high vacuum. Therefore, by the MBE process, an oxide superconductor thin film can be deposited without unnecessary interface potential so that superconducting current with little turbulence flows through the oxide superconductor thin film. In addition, it is possible to form a sharp superconducting junction such as a Josephson junction by the MBE process.
The MBE processes can be classified into two kinds. One is so called a co-evaporation method in which all the molecular beams are sent simultaneously so that all the materials are supplied concurrently. Another is so called a sequential evaporation method in which each of the molecular beams is sent in a predetermined order so that a thin film is deposited layer by layer.
Since an oxide superconductor is a plural compound and has a layered crystalline structure, the sequential evaporation method is suitable for forming a high quality oxide superconductor thin film having a smooth surface. In a prior art, if an oxide superconductor thin film is deposited by the co-evaporation method, the oxide superconductor thin film has a rough surface and poor quality.
However, the oxide superconductor is composed of plural constituent atoms so that plural vapor sources and molecular beam sources are necessary to deposit an oxide superconductor thin film by an MBE process. In addition, the oxide superconductor has a layered crystalline structure which is composed of many atomic layers. Therefore, when an oxide superconductor thin film is deposited by the sequential evaporation method, vapor sources and molecular beam sources are controlled so as to supply molecular beams in the predetermined order. It might be difficult to precisely control intensity and emitting durations of plural molecular beams in the sequential evaporation method, so that it takes a lot of time and labor to form an oxide superconductor thin film by the sequential evaporation method.
Now, the present invention will be explained with reference to the accompanying drawings.